1. Field of the Invention
The present invention relates to a one-way clutch used in lubricating oil to which organic molybdenum is added.
2. Related Background Art
A one-way clutch includes outer and inner races which are rotated relative to each other and is constructed so that rotational torque is transmitted in only one direction by engaging sprags or rollers for transmitting the torque between the outer and inner races with a cam face provided on a track surface of the outer or inner race and that the clutch is idly rotated in an opposite direction.
In general, a clutch is used as a torque converter or a transmitting mechanism of an automatic transmission of a vehicle and as a starter of a motor bicycle. Among one-way clutches used as the starter, as a lubricating condition, there are one-way clutches in which the one-way clutch is lubricated in common with the lubricating of the engine. In this case, the engine oil is used to lubricate the one-way clutch.
By the way, in recent years, in order to reduce friction for the purpose of achieving low fuel consumption, engine oil to which organic molybdenum is added has been used. The reason is that the organic molybdenum represented by molybdenum diocarbide (MODTC) has the effect for considerably reducing friction between metal parts.
However, if the engine oil to which the organic molybdenum is added is used as the lubricating oil for the one-way clutch as well as the engine, there arises a problem that an engaging service life of the one-way clutch is reduced, with the result that the clutch cannot maintain engagement, thereby making it impossible to transmit the torque.
Regarding the sprag which is one of constructional members of the one-way clutch, since the sprag as a torque transmitting member is subjected to great face pressure during the transmission of the torque, the sprag is formed from bearing steel such as SUJ2 material, or is formed from heat-resistive bearing steel such as M50 material if the sprag is used under a high temperature environment. If such a one-way clutch of sprag type is used with the lubricating oil same as the engine oil, since the sprag is subjected to the engagement and the idle rotation repeatedly during use, the engaging surfaces of the one-way clutch are engaged with high face pressure repeatedly under a high temperature lubricating condition.
In this case, if the organic molybdenum is added to the engine oil, since heavy metal such as Cr included in the SUJ2 material and the M50 material constituting the sprag and molybdenum (Mo) greatly included in the M50 material belong to 6A group transition metal same as molybdenum of the organic molybdenum added to the lubricating oil, affinity becomes strong. In particular, it is considered that, as is often the case with the sprag, if the same cam face area is exposed to the high face pressure, the adhesion of the organic molybdenum onto the cam face of the sprag is promoted, with the result that the organic molybdenum adhered to the cam face of the sprag is decomposed to form a surface coating or film mainly including molybdenum disulfide (MOS2) on the friction surface, thereby reducing the coefficient of friction to generate slip.
In the engine oil to which the organic molybdenum is added, it was ascertained that, when a stroking endurance tests with repeated load were performed at a temperature of 130° C., the sprags of the one-way clutch cannot engage with the outer or inner race at certain cycles or more.